Induction heating has been widely adopted in domestic, industrial and medical applications. Induction heating refers to the technique of heating an electrically conducting object (such as a metal) by electromagnetic induction whereby electric current is generated in a closed circuit (the object) by the fluctuation of current in another circuit placed physically close to the object. For example, an induction cooker includes a resonant tank driven by an alternating current to induce an alternating magnetic field at an induction coil. The alternating magnetic field at the induction coil induces current in a metal cooking pot placed physically near the induction coil. The current induced in the resistive metal cooking pot generates heat which in turn heats the food in the cooking pot.
A commonly used topology for induction heating applications is the single switch quasi-resonant inverter topology including a single power switch and a single resonant capacitor to supply variable resonant current to the induction coil. The single switch quasi-resonant inverter is often implemented using an insulated gate bipolar transistor (IGBT) as the power switching device due to the high power capability and high switching frequency operation of IGBTs.
Overvoltage conditions, such as a power surge, can be a serious problem for the single switch quasi-resonant inverter circuit. In particular, the power switching device in the quasi-resonant inverter circuit may fail or become permanently damaged when a voltage exceeding the voltage rating of the power switching device is applied. For example, an abnormally high surge voltage may be applied to the AC input line during a lightning event. In the event that the surge voltage exceeds the breakdown voltage of the power switching device, the power switching device may become irreversibly damaged if remedial action is not taken within a very short time from the power surge event, on the order of a few microseconds.
Furthermore, during the normal operation of the single switch quasi-resonant inverter, the power switch is turned on and off at a given switching frequency, for example, 30 kHz. During the time the power switch is turned off, the voltage across the power switch can reach a high peak voltage, such as 1 kV. If the power switch is unexpectedly turned on again when the voltage across the switching device is still at a high voltage value, the power switch experiences hard switching which can negatively impact the efficiency and reliability of the power switch.